The present invention relates generally to methods for the production of carbon nanotubes, and more particularly to methods for the production of carbon nanotubes as free-standing films or nanotube mats by the thermal decomposition of transition metal complexes.
Carbon nanotubes, first reported by Iijima, have metallic electrical conductivity in the arm-chair configuration and are semi-conducting in a zig-zag structure. Because of the unique physical and chemical properties arising from their structure and size, they have potential applications in devices requiring efficient field emitters, high-strength fibers, strong radiation shields, energy absorbing materials, nano-scale catalytic beds, efficient gas storage, nano-circuits, nano-scale transistors, charge storage materials and white light sources. In addition, they also have potential applications as nano-sized, frictionless bearings, in the construction of nano-mechanical devices, electromechanical devices and opto-electronic devices. For most of these applications, it is desirable to synthesize nanotubes of high purity over a large area in film form. Control of diameter, morphology and lengths of the carbon nanotubes produced is also a useful requirement for practical applications of a particular process. In addition, simplicity and flexibility in the production and fabrication of carbon nanotubes will make a process commercially attractive.
Available carbon nanotube synthesis methods include arc discharge, laser ablation, and chemical vapor deposition (CVD) processes and variations of all these procedures. While the first two methods involve the breakage of graphite to carbon clusters and re-building of carbon nanostructures, the CVD process builds up the carbon nanotubes starting from smaller components. This difference directly relates to the efficiency of the processes with the CVD process being more efficient. Some of the reported CVD processes are economical, efficient and said to result in oriented films of nanotubes with the possibility of control of their diameter and lengths. The CVD and related processes involve the use of a metal catalyst (usually a transition metal) either pre-made on a support or made by in situ reduction of a suitable precursor and a carbon source in the form of a gas or vapor at high temperatures. Alternatively, a mixture of a carbon source (such as benzene, toluene or xylene) and a transition metal catalyst (such as ferrocene or nickelocene) as a metal source is pyrolyzed to produce nanotubes. These two general methods need high temperatures (typically >1000° C.) to produce the necessary catalytic particles.
Some of the drawbacks in the existing methods can be summarized as follows: 1. Lack of control/determination of metal (catalyst)/carbon (precursor) ratios in the starting materials and/or in the products. 2. Required fabrication of specialized apparatus. 3. Limited area coverage of nanotube mats. 4. Control of morphology of the produced nanotubes. 5. Lack of control in the yields of undesired side products, such as carbon onions and amorphous carbon. 6. High temperatures involved.
Accordingly it is desirable to provide an improved method of producing carbon nanotubes, particularly as free-standing films or nanotube mats. It is also desirable to provide a carbon nanotube fabrication method which reduces or eliminates the prior art process problems described above.